Prior pressure transducer assemblies exist in which a semiconductor pressure transducer die is mounted on a base substrate within a cavity to which vacuum pressure is to be applied. In such a prior configuration wire bonds are used to provide electrical connection between bonding pads on a top surface of the semiconductor pressure transducer die and conductor paths on the substrate.
In assemblies such as those described above, there is typically a substantial concern about contaminants affecting the operation of the pressure transducer by attacking the semiconductor chip metalization and/or the bonding wires or substrate conductor metalizations. For example, when the pressure transducer comprises an automobile pressure transducer used to measure manifold pressure, typically the entire assembly will be exposed to an atmosphere which may contain gasoline and mild acids as contaminants in the vacuum pressure to be measured by the transducer. These contaminants may also be external to the cavity in which the transducer is mounted wherein the external contaminants may seep into the cavity. Also, when the walls surrounding the pressure transducer are soldered to the base substrate on which the transducer is mounted, this can provide a source of other contaminants due to flux used in the soldering operation which was not completely removed and/or the presence of bubbles in the solder connection which will be drawn from the solder connection into the cavity during the application of vacuum pressure.
To provide some protection against all of these contaminants, one known prior art assembly shown in FIG. 2 has applied, as an encapsulation material, a conventionally available polyfluorosiloxane oligomer comprising Shin Etsu FE-53, which is available from Shin Etsu Silicone and comprises a polyfluorosiloxane gel. This material, when cured, has some resistance to attack by gasoline. Other types of known conventional die encapsulation materials, including known thixotropic die encapsulation materials, do not sufficiently resist attack by gasoline and were therefore found to be unsuitable for use in an automotive environment. Typically a drop of the uncured fluorosiloxane polymer (oligomer) is placed on the semiconductor die and then several additional drops are placed in the cavity in which the die is mounted. Then the oligomer is cured and the structure shown in FIG. 2 is the result. However, such a structure typically leaves portions of the semiconductor bonding wires exposed. This subjects these bonding wires to electrochemical attack (corrosion) by contaminants, and also these wires then provide a pathway for acids and contaminants to diffuse along the wires to the die surface where, in time, they attack the aluminum bonding pads on the die and cause the pressure transducer to fail.
A proposed solution to the above stated problems of the prior art assembly is shown in FIG. 3 in which the same conventional polyfluorosiloxane material is used to substantially fill the internal cavity in which the pressure transducer is mounted. However, this type of solution has resulted in a substantially different problem which also impairs the operation of the transducer. For the assembly shown in FIG. 3, when vacuum pressure is applied, bubbles and flux contaminants from the solder connection between the surrounding walls and the base substrate on which the semiconductor die is mounted will be drawn into the encapsulation material comprising the polyfluorosiloxane gel. These bubbles and contaminants are mobile in the encapsulation material and can travel to the bonding sites of the wire bonds on the top surface of the semiconductor die or the bonding sites on the conductor paths on the base substrate. These bubbles and contaminants can affect the operation of the pressure transducer such that erratic results may be obtained due to deterioration of the electrical connections to the die. The bubbles can also interfere with the pressure transducer properly measuring applied vacuum pressure since they will provide an erratic coupling of the vacuum pressure to be sensed to the top surface of the pressure transducer semiconductor die.